The present invention relates to a dry unit of a dry end, in particular, of a paper machine for producing paper webs and including at least one drying cylinder and a dry felt, which is at least partially wrapped around the drying cylinder, with a paper web passing between the dry felt and the drying cylinder.
A dry unit of the above-described type is disclosed in German publication DE 44 16 585. The publication discloses a dry unit for a dry end and including a plurality of heatable drying cylinders and pressure bands (also called dry felts) which press a to-be-dried material web to a drying cylinder.
Generally, single-row and double-row dry units are used. With a single-row dry unit, a material web passes, along a meander-shaped path, alternatively about a drying cylinder and deflection rolls. Thus, when a single-row dry unit is used, only one side of the material web comes into contact with outer surfaces of the drying cylinders, with the other side abutting the pressure band.
With a double-row dry unit, a material web likewise passes along a meander-shaped path, but it is passed, alternatively, from a drying cylinder of one row to a drying cylinder of another row so that one side of the material web comes into contact with the drying cylinder of the one row, and another side of the material web comes into contact with the drying cylinder of the another row.
Usually, each of the two drying cylinders cooperates with a respective pressure band which presses the material web to the outer surface of the drying cylinder.
A dry end of a paper machine often includes a combination of single-row and double-row dry units, whereby the characteristics of a material web, such as shrinkage and strength, can be appropriately influenced.
An increase of the material web strength can be achieved by providing, in an interior of the material web, a temperature of about 100.degree. C. To this end, often gas-fired drying cylinders are used, the temperature of which can reach about 300.degree. C. By additionally using high-stressed, rigid pressure bands, a vapor pressure in the inside of the material web can be attained that would be much higher than the environmental pressure.
However, at that, upon lifting of a pressure band from the material web surface, the vapor pressure established in the material web interior is released abruptly. The sudden release of the vapor pressure results in splinting of the material web in some places or, in worst cases, the sudden release of the vapor pressure leads to rupture of the web.
Accordingly, an object of the invention is to provide a dry unit in which the splinting of the material web and/or web rupture is reliably prevented.